1. Field of the Invention
The present invention relates to a microwave oven, and more particularly to a microwave oven adapted to radiate microwaves of mutually reverse phases to minimize impedance variation of a waveguide in response to load change of foodstuff, thereby maintaining an output of the microwave at a constant level regardless of load amount of the foodstuff and maintaining an electric field distribution in a cavity at a constant level as well.
2. Description of the Prior Art
Generally, a microwave oven is used for radiating microwaves generated by oscillation of a magnetron into a cavity via a waveguide to cook foodstuff lying on a predetermined position in the cavity by way of dielectric heating.
FIG. 1 is a schematic sectional view of a waveguide in a microwave oven according to one embodiment of the present invention, and FIG. 2 is an interpretation drawing of injection struction of the waveguide in FIG. 1, where the waveguide 1 is formed with an insertion inlet 9 through which an antenna 3a of a magnetron 3 is inserted and a rectangular radiation hole 7 through which microwave are radiated into a cavity 5.
The microwaves produced by oscillation of the magnetron 3 are radiated into the cavity 5 through the waveguide 1 to cook the foodstuff inside the cavity 5 by way of dielectric heating.
As illustrated in FIG. 2, if a power of the magnetron 3 is given as P.sub.in and a power at a predetermined position in the cavity 5 is defined as P.sub.out, the output P.sub.out can be obtained by following formulae 1, 2 and 3. EQU P.sub.in =E.sup.2 .sub.s [Formula 1] EQU E.sub.y =E.sub.s sin(.chi.) [Formula 2] EQU P.sub.out =(E.sub.y).sup.2 l =(E.sub.s sin(.chi.)).sup.2 =E.sub.s.sup.2 sin(.chi.).sup.2 [Formula 3]
Where, E.sub.s is an electric field energy (by way of example, input electric field energy) formed by microwaves produced by oscillation of the magnetron 3 and E.sub.y is an electric field energy (by way of example, output electric field energy) formed at a predetermined position in the cavity 5. The power of magnetron 3 is a squared value of E.sub.s formed by microwaves generated by oscillation of the magnetron.
Furthermore, the microwaves generated by oscillation of the magnetron 3 are sine waves of certain phase so that the electric field energy E.sub.y at a certain position in the cavity 5 is the electric field energy E.sub.s multiplied by sin(.chi.), and P.sub.out is a squared value of E.sub.y.
Accordingly, the power P.sub.out varies according to load change. FIG. 3 is a polar chart where impedance characteristic of waveguide 1 according to load change of the foodstuff is illustrated. FIG. 3 is based on a microwave frequency range of 2.44-2.47 GHz, with a load of 2,000 cc water, 500 cc water and 100 cc water, respectively.
As illustrated in FIG. 3, in case of a load of 2,000 cc water, a voltage standing wave Ratio (VSWR) becomes large. In other words, impedance of the waveguide 1 becomes small to increase the power of a microwave oven. In case of a load of 100 cc water, a Voltage Standing Wave Ratio (VSWR) becomes small. In other words, impedance of the waveguide 1 becomes large to thereby decrease the power of the microwave oven.
In other words, there is a problem in that, when a load of foodstuff is large, the power of the microwave oven is a little bit high but when the load is small, impedance of the waveguide is increased to thereby decrease the output of the microwave oven.
Furthermore, there is another problem in that impedance of the waveguide 1 is varied too much by variation of load of foodstuff to thereby make electric field distribution in the cavity 5 inconstant.
There is still another problem in that one waveguide 1 cannot be applied to various kinds of cavities 5, so that each cavity 5 needs separate waveguide 1.
To overcome these problems, Japanese laid-open patent No. Hei 6-111933(disclosed on Apr. 22, 1994) is disclosed, where a two-way guide system of microwave oven, as illustrated in FIG. 4, includes an upper and a lower radiation hole 11a and 11b, a cavity 12, a magnetron 14 for generating via an antenna 13 microwaves having .lambda.g frequency, and a waveguide 15.
At this time, electric waves generated from the magnetron 14 serve to form voltage standing waves, which in turn are radiated into the cavity 12 via the radiation holes 11a and 11b to evenly heat the foodstuff.
However, there is a problem in the conventional waveguide system of a microwave oven thus constructed, in that only the dispersion efficiency of the microwaves is made better, so that power variation of the microwave oven cannot be overcome adequately according to load changes of foodstuff.
Another prior art of Japanese laid open patent No. Hei 4-233188 (disclosed on Aug. 21, 1992) is disclosed, where a microwave oven for two-way heating method includes, as illustrated in FIG. 5, a waveguide 19, an upper and lower radiation hole 21a and 21b, a magnetron 23, an antenna 25, and a protruder 27, where the protruder 27 is constructed to have almost the same width as that of the distance of the antenna 25.
At this time, the radiation holes 21a and 21b are so formed as to have maximum distances and the waveguide 19 is formed at an upper side thereof with a horizontal surface 19a and is formed at a bottom side thereof with a slant surface 19b.
In the two-way method of a microwave oven thus constructed, microwaves generated from the magnetron 23 are radiated via the antenna into the waveguide 19 and the microwaves radiated into the waveguide 19 form voltage standing waves via the protruder 27 to be directly radiated to the cavity 17 via the upper radiation hole 19a. Part of the voltage standing waves are radiated slantedly via the lower radiation hole 19b to evenly heat and cook the foodstuff laid on a floor of the cavity 17.
Here, a structure theory of the waveguide 19 for reverse phase radiation can be obtained by the formula 4. EQU A-B=(K+n.multidot.0.5).lambda.g [Formula 4]
Where, A=an overall length of the waveguide 19 measured from an upper periphery of the upper radiation hole 21a to a lower periphery of the lower radiation hole 21b, B=length of the waveguide 19 measured from a central axis line 29 to an upper periphery of the upper radiation hole 21a, K=a constant of value against a 0.7-0.9 range, n=0, 1, 2, 3 . . . and .lambda.g=wavelength of a basic mode for a waveguide 19.
The length by the formula 4 is a function of .lambda.g, and microwaves of mutually different reverse phases(+, -) serve to evenly heat and cook the foodstuff lying on a floor of the cavity 17.
However, there is a problem in the conventional two-way method of microwave oven thus constructed in that an output waveguide is long and thick to make it difficult to accommodate electronic elements, and impedance of the waveguide 19 is inconsistent according to the cavity 17, so that, whenever the cavity 17 is changed in size thereof, sizes and positions of the upper and lower radiation holes 21a and 21b are inevitably adjusted and redesigning is unavoidable.
Still furthermore, there is another problem in that microwaves in the cavity 19 are radiated into the cavity with phase differences, electric field distribution mode in the cavity 17 is not wholly formed by the upper and lower radiation hole 21a and 21b but formed chiefly at the upper and lower.